simpler decode if cofactor 4

aux/ristretto.sage

@@ -167,22 +167,36 @@ class RistrettoPoint(QuotientEdwardsPoint):
         a,d = self.a,self.d
         x,y,z,t = self.xyzt()
         
-        u1    = a*(y+z)*(y-z)
-        u2    = x*y # = t*z
-        isr   = isqrt(u1*u2^2)
-        i1    = isr*u1
-        i2    = isr*u2
-        z_inv = i1*i2*t
+        if self.cofactor==8:
+            u1    = a*(y+z)*(y-z)
+            u2    = x*y # = t*z
+            isr   = isqrt(u1*u2^2)
+            i1    = isr*u1
+            i2    = isr*u2
+            z_inv = i1*i2*t
         
-        if self.cofactor==8 and negative(t*z_inv):
-            if a==-1: x,y = y*self.i,x*self.i
-            else: x,y = -y,x # TODO: test
-            den_inv = self.magic * i1
+            if self.cofactor==8 and negative(t*z_inv):
+                if a==-1: x,y = y*self.i,x*self.i
+                else: x,y = -y,x # TODO: test
+                den_inv = self.magic * i1
+            else:
+                den_inv = i2
+
+            if negative(x*z_inv): y = -y
+            s = (z-y) * den_inv
         else:
+            u1    = a*(y+z)*(y-z)
+            u2    = x*y # = t*z
+            isr   = isqrt(u1*u2^2)
+            i1    = isr*u1
+            i2    = isr*u2
+            z_inv = i1*i2*t
             den_inv = i2
 
-        if negative(x*z_inv): y = -y
-        s = (z-y) * den_inv
+            if negative(x*z_inv): y = -y
+            s = (z-y) * den_inv
+            
+        
         return self.gfToBytes(s,mustBePositive=True)
         
     @classmethod